1. Field of the Invention
The present invention relates to a two-dimensional scanning apparatus usable in an image displaying apparatus of a scanning type capable of displaying a two-dimensional image by scanning light in a two-dimensional manner.
2. Related Background Art
There have been proposed various two-dimensional scanning apparatuses in which a light spot is scanned in a two-dimensional manner, and a two-dimensional image is formed due to the effect of its after image.
It is generally known that distortion is likely to appear in a two-dimensional image formed on a surface to be scanned (a scanned surface) when a light beam is two-dimensionally deflected and scanned. The distortion includes trapezoid distortion, uniform-velocity scanning distortion, linear scanning distortion, and so-called TV distortion of curvature in an image frame formed on the scanned surface.
Japanese Patent Application Laid-Open No. H8(1996)-146320 discloses a two-dimensional scanning apparatus in which a light beam emitted by a light source is two-dimensionally deflected by a deflecting unit, and a light spot is two-dimensionally scanned through a scanning lens having f-θ characteristics as distortion characteristics to form an image. It is described therein that f-θ characteristics of the scanning lens and electrical correction can correct the TV distortion that is distortion appearing in an image formed when light is two-dimensionally scanned. Further, Japanese Patent Application Laid-Open Nos. H11(1999)-84291 and 2001-281583 disclose a two-dimensional scanning apparatus in which an optical element having a refractive surface and a reflecting surface is used, an optical path is folded in the optical element, and inner and outer refractive surfaces or reflecting surfaces are comprised of rotational asymmetrical surfaces having no rotational symmetrical axes to correct decentering aberrations.
In those two-dimensional scanning apparatuses, velocity uniformity of scanning light on the scanned surface is preferably corrected over a wide scanning angle though the apparatus is constructed using a single optical element. Further, the apparatus is capable of achieving telecentric characteristic that is necessary for highly precise drawing.
In the two-dimensional scanning apparatus disclosed in Japanese Patent Application Laid-Open No. H8(1996)-146320, however, it is actually difficult to electrically correct the TV distortion though the apparatus aims at correcting the TV distortion by the f-θ characteristics of the scanning lens and the electrical correction.
The two-dimensional scanning apparatuses proposed in Japanese Patent Application Laid-Open Nos. H11(1999)-84291 and 2001-281583 do not aim at correcting the TV distortion form the beginning. Further, it is difficult to stably correct the TV distortion in those two-dimensional scanning apparatuses since the optical element is provided with a reflecting surface having optical power and its surface precision is very severe. Furthermore, the thickness of the optical element tends to increase since a folded optical path needs to be established in the optical element. Moreover, when the optical element is formed of plastic material, large influences of distribution of inner refractive index and double refraction are likely to occur.